Improve ai_ckeditor UX when no provider modules are installed
## Summary
`ai_ckeditor` currently assumes that at least one provider module is installed. If no provider module (such as `ai_provider_openai`) is available, the CKEditor AI functionality does not work and may result in confusing behavior or errors.
The module could provide clearer feedback to site builders or explicitly require provider modules.
## Problem
`ai_ckeditor` can be enabled without installing any provider modules. From a site builder perspective, the module appears to be configured correctly because the CKEditor buttons are available, but attempting to use them fails.
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This can be confusing because there is no indication that a provider module is required.
Site builders who are new to the AI ecosystem may not realize that `ai_ckeditor` depends on having at least one AI provider available.
## Proposed solution *(optional)*
One of the following improvements could help:
- Display a clear message in the configuration UI when no provider modules are installed.
- Prevent the CKEditor AI actions from being used until a provider is available.
- Add an explicit dependency on provider modules.
- Show a helpful error message explaining that a provider module (for example, OpenAI) must be installed and configured.
This would make the initial setup experience easier and avoid unnecessary debugging.
## Workaround *(optional)*
Install and configure a provider module such as `ai_provider_openai`.
## Affected modules / components *(optional)*
- `ai_ckeditor`
- Provider modules (`ai_provider_openai`, etc.)
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